The present invention is directed to threshold controlled circuits with hysteresis. In particular, the hysteresis precedence circuit of the present invention may be used with undervoltage lockout circuits.
There are numerous circuit applications in which a comparator compares a control voltage with one of a number of thresholds. For example, U.S. Pat. No. 4,887,022 (Gontowski) discloses an undervoltage lockout circuit having a first high threshold which must be reached before the circuit will turn on. Hysteresis is activated so that a second lower threshold is put in place after the supply voltage reaches the first U.S. Pat. No. 4,887,022 can run into problems if control of the supply voltage causes it to drop before the second threshold is activated. This would make the circuit latch somewhere between the two thresholds.
It is an object of the present invention to ensure that the hysteresis will be activated before the control circuit affects the supply voltage. Hysteresis precedence is advantageously ensured by the present invention without the use of time delay circuitry.